Electric furnace



April 23,1946.- A. L, WEYHING ELECTRIC FURNACE Filed Feb. 14, 1940Sheets-Sheet 1 Aprily 23, 1946.` A1., wEYl-HNG-V ELECTRIC FURNACE 2Sheets-Sheet 2 Filed Feb. 14, 1940 W M A mm WW T. NM N E .R V n mL T TJA w Patented Apr. 23, 1946 ELECTRIC FURNACE August L. Weyhing,Louisville, Ky., assignor to Edmund A. Steinbock, Louisville, Ky.

Application February 14, 1940, Serial No. 318,900

1 Claim.

This invention relates to improvements in furnaces, and particularlyelectric furnaces.

The furnace of this invention was developed primarily for use in smallproduction laboratories such as dental laboratories and the like, whichare usually located in office buildings and other locations where draftchimneys or stacks are not available and cannot be readily constructed.Another use for these furnaces is where a relatively small and portablefurnace is desirable or necessary, since they may necessarily be shiftedfrom position to position.

It is one of the chief objects of this invention to provide a furnacefrom which the fumes and products of elimination caused during operationof the furnace can be readily and completely disposed of withoutcontaminating the atmosphere of the room in which the furnace is used.

Another object of this invention is the provision of an electric furnacewhich must generate a relatively high heat and in which deleteriousgases are generated and prevented from attacking the heating elements.

Another object of this invention is the provision of means in a furnacewhich is used for burning out wax from molds to prevent the wax frominterfering with or breaking down the heating elements.

A still further object of this invention is the provision of an electricfurnace for heating molds made of investment compositions and in which aneutral or oxidizing atmosphere is maintained to avoid breaking down ofthe mold.

A still further object of this invention is the provision of a furnacein which gas or the like may be introduced during its operation, or inwhich a partial or semi-vacuum may be established, depending upon thecontent of the furnace and the result desired.

Other objects and advantages of the present nvention should be readilyunderstood by reference to the following specication when considered inconnection with the accompanying drawings and forming a part thereof,and it is to be understood that any modifications may be made in theexact structural details therein shown, without departing from orexceeding the spirit of the invention as defined by the appended claim.

In the drawings:

Fig. l is a front elevation of an electric furnace embodying theimprovements of this invention.

Fig. 2 is a side elevation of the furnace shown in Fig. 1, as seen fromthe right hand side thereof, parts being broken away to clearlyillustrate the interior construction of the furnace.

Fig. 3 is a rear, elevational view of the furnace as shown in Fig. 1.

Fig. 4 is an enlarged, fragmentary, vertical, sectional view taken online 4 4 of Fig. 3.

Fig. 5 is an enlarged, vertical, sectional view taken at right angles ofFig, 4, on line v5-5 of Fig. 2.

Fig. 6 is an enlarged, fragmentary, horizontal, sectional view taken online 6--6 of Fig. 3.

Fig. 7 is an enlarged, vertical, sectional view, taken on line 'l--l ofFig. 3. Y

Throughout the several views of the drawings, similar referencecharactersare employed to denote the same or similar parts.

As was noted above, this furnace was primarily developed for use indental laboratories and similar places for the burning out and heatingof molds made from investment compositions and which laboratories aregenerally situated in office buildings, where access to exhaust stacksand the like are impossible. As is well known, metals, whether in thepure state or alloyed, have considerable shrinkage when cooling from thefluid state to the solid state after being cast. In dental work, andwhenusing a gold alloy as bridges, fillings, plates, and the like, alinear shrinkage of 1.25% is the rule. To overcome this shrinkage, themold investment composition is composed generally of some form ofplaster, a ller, a time setting element and a thermal expanding agent,together with other ingredients to provide the mold with certaincharacteristics. The mold has embedded :therein a wax pattern madeaccurately to the size and shape of the dental part to be made. The moldis placed in a furnaceand heated rst to dissipate the wax pattern bymelting, and burning, and then the mold is heated to cause the thermalexpansion agent to expand the cavity left by the disappearing waxpattern an amount equal to the metal shrinkage upon cooling from thefluid state to the solid state. As can readily be appreciated, themelting wax would normally flow onto the heating element embeddingsurface, which is more or less porous, and would nd its way to theheating elements themselves, and in a short time damage said elements.At the same time, the constituents of the mold, during the thermalexpansion thereof, give off gases which may be deleterious to theheating elements and to :the membranes of the head and throat, if notdownright poisonous to life itself, would permeate the laboratory roomif not properly and completely exhausted from the furnace to theatmosphere.

It has been found that a furnace, without the improvements disclosed inthis application, burned out in ve runs, where the furnace had beenheated up to 1300o F.'for gold alloy castings and to 1700 for stainlesssteel castings; that is, the

heating elements were destroyed to the extent that the furnace wasuseless, while, with the improvements of this application, the furnacewas operated continuously for months without in videntied by, referencecharacter 35. ,toI maintain the Vdoor in its open position, use is thefront frame I is a rear frame I3, similarin all respects to the frontframe I0 and having.

depending rear legs I4 and I5. A sheet`metal housing I6 extends betweenthefront and rear frames I0 and Il3, and comprises aV top/I1 andopposing sides I8. The sides I=8 are joined by means of a sheet metaltray I9 at a point Ajust above the end frame legs. The top, 4sides andbottom ofthe furnace are similarly constructed and'compris'e affirstlayer 20 of heat-insulating materiala second layer 2| of heat insulatingmaterial and a final inner lining 22 of refractory material.Theseseveral layers are formed initially in blocks"orfslabs andsupported one on 'another with the inner liners interlocking and formingthe supports for the vwhole and the'proper spacing ofthe inner `cavitytop, bottom and yside walls. The front wall 23 and rear wall 24 aresimilarly construetedand likewise made up of preformed elements, theoutermost 24a being compressed asbestos material in such a manner as t'oprovidea slate like outer surface, and which contacts the end edges 4ofthe outermost insulation layer'Z. Behind the outermost layer gfla Yisalayer 2'5 vof heat refractory material,

""sruchasn'febrick, which contacts the edges vof 'the second heatinsulating layer 2I and the inner refractory liner `,22. The yfront wall23 has its layers-24a and v2`5p'r`ovided with a rectangular openingf26whichaligns'with the inner surfaces of the inner Vliner22 and'providesanopening to thehfrnaemuiile chamber. Y

Tleope'ning 26in'the` front wall of the furnace is clcsedjb'y the usualswinging 'door 21, which, as 'ulsialjcomprises an louter sheet metalcasing VY28, 'vvitl 1 Ya suitable heat resisting material-'29 llli'ng.Securedto `thelower `ends lof the 'door sides v"are arms f30 Vhavingapertures below,

thrug'h whichpas'ses'an aXlebar3I. The axle bar 3`I'is secured in Ythedoor arm bearings by any suitable means -`and Athe axle bar passes'thr'igh apertures formed in angle iron brackets '32 'andf33vwlich'ha've their 'opposite ends respectivelysecured to the front frameI0. The axle bar'30'projec't`s to' onesde of the furnace proper and "hassecured thereto or integral therewith 'a counterwe'ight` 34 employed tohold the door in the closedfposition, as illustrated in the drawings.The doorfhowever, may be opened or positioned at rightjangles to thatillustrated in solid linesjin the drawings,and this position isillustrated by dot and dash lines in Fig. 2, and In order madehof anabutment plate 36 which convenientlywtakeus themform of adslhort pieceof angle iron, suitably secured to vthe front of the furnace.

, ,Tlle'mief. linings 2.2 A0f the .furnace palper orV the muiile, asnoted above, arehof refractory material, and they have secured oryembedded therein the heating wires or elements, and these elements havetheir ends projecting from the refar end of the linings. Theseprojecting ends, indicated by reference character 31, project through-the'rear wall 24 of the furnace. By this construction, adjacentsides ofthe mufe liners 2 2jhave their heating elements 'exteriorly of thefurnace Y adjacent 'one v:another [and fare:

connected by"'a'suitable connector. By this form sides for the rheostatcompartment, while the front of this compartment is formed by a plate 39suitably secured to an inner surface of the front leg front flanges.Projecting through the front 39 is a shaft 40 carrying exteriorly of theYplate -39 a iknob 4I.` Within the `rheostat compartment y42,*'1-,hesh'aft 46 is journale'd'in a bracket 143 andhas secured thereto a bevelgear or `piniong44 meshing with -a gear or pinion v45 carried by Itherheostat'shaft 45. The shaft 46, vas is `Ausual, is journaled lin therheostat casing V li'l and carriesamovable 'control arm 48 which isadapted to lpass over the resistance buttons A(notsh'own) Ywhich havethe electrical-resistance conr'iected therewith. The rheostat'casing 41is secured to the undersurface of the furnace bottom I9 in any suitablemanner, such, for example, as by straps or arms 43.

The rheostat hasrthe usual contactfsupportving vplate 55 lfor bindingposts V`5I vand Y52 which Vare respectively connected withthe resistance'58 Vof the electric source or commercial Voltage is connected by meansof wire 59 with another heatingelementjconnector 38. As will be readilylseen, this vwiring makes a complete electrical Acircuit from ltheelectric source or commercial voltage, by wayY o f -wrire51 to wire "54,rheostat, wires 53,V heatinglelements and their connectors toV wirey 59v;and"wire'58 backto the electrical source orcommercial voltage.

The back Ywall 2,4 ofythe furnace is provided 'with an` aperture 50llocated substantially centrally of the width ofthe muffle chamberand'substantially at the top of the Vertical height thereof. "Securedtothe outer surface ofthe back Wall 24o-is anfapertured `member 6I whichconveniently takes the formof a standard pipe flange andhaving a centralthreaded aperture in axial alignment with thegaperture 60 Yin the backwall 2 4.U Threadedfintothe pipe flange 6I is a nipple vv62 whichhasitsotherend threaded into apipe T vB3. Extendiri'gjfrm thel upper sideof the T 63 is a pipe 64 which,-as^wil1'later be 'made clear. 1s theexhaust pipe for the furnace. VThe lower side lof the vT63 'has threaded'thereinto a plug '65 whichis centrally bored or apertured, alsat 6,6.It might be notedoatthis time that the aperture 65 provides aclrainA forthe moisture 'drawn Vinto the vexhaust pipe '64 and that the 'exhaustpipe 64,' which'is` nothing more than vordinary iron pipe, extends totasuitableV point in 'the wall of the building or room, `such 'as a,window, 'where it is projected into the atmosphererfor dissipated gasesand '.thevlike. Letintothe T Ymember'63,

through onesi'de thereofjat the uppermost point above the aperture 68,is a tube 61 having its inner end 68 upwardly disposed into the exhaustpipe 64. In order to secure the exhaust pipe 68 in the side of the T 83,use may be conveniently made of the usual tubing connection 68. Theother end of the tubing 61 is connected to one side of a T 10, the otherend of which T 'el is connected by a piece of tubing 1i with an airpressure gauge 12. The third opening of the T 18 has connected therewithone end of a tube 13, the other end of which is connected to anadjustable air control valve 14. The air control valve 14 has its otherside connected with a pipe 15 which extends from an air pressuregenerator, tank or the like.

The air control valve 14 may be cf any convenient or desirable form, onesuch valve being shown, in Fig. 6, and comprising a valve body 1,6provided with a valve seat 11 with which a needle point 18 co-operates.The needle point valve 18 has a threaded stem 19 projecting therefromand passing through a packing nut 80 carried by the valve body 15. Theouter end of the valve stem 18 has secured thereto knob 8l by which thevalve is adjustable.

The valve 14 is disposed behind an instrument pane1 82 which is securedto and upstands from the rear end of the furnace top I1.

Any suitable means may be employed for positioning the valve that shownincluding a short metal strap 83 through which the air supply pipeY 15passes and is welded as by welds Se. One end of the strap 83 is providedwith a threaded aperture through which a screw 85 passes for clampingthereto a flange 86 rearwardly projecting from one lateral Side of theinstrument panel 82. It should be noted that the air pressure gauge 12is secured to the panel 82 and is conveniently located adjacent the airregulating valve 14 so that the pressure in the tube 68 can be readilydetermined and adjusted.

Formed through the rear wall 24 of the furnace at one of the uppercorners of the mulile chamber is a second aperture 81 in which isdisposed a thermocouple 88. In order to protect the thermocouple, it isencased in a substantially non-porous refractory covering 89 which hasits inner end closed as at 98 and its outer end open to permit the endsof the thermocouple to project therefrom. The thermocouple is acontinuous wire member bent in the middle and having its ends outwardlyof the rear of the furnace. Said rear ends of the thermocouple will benddownwardly parallel with the back of the furnace and are respectivelyindicated by reference numerals 9i and 82, respectively connected withbinding posts 83 and 84 carried by the furnace back wall. The bindingposts 93 and 94 are respectively connected by wires 95 and 96 withbinding posts at opposite sides of a pyrorneter 98 carried by theinstrument panel 82.

In order that the air gauge and pyrometer may be readily read, theforward face of the instrument panel 82 carries a suitable light 99arranged to direct its rays onto the dials of the said pyrameter and airgauge.

In operation, the exhaust pipe 64 is connected to atmosphere while thepressure pipe 15 is connected to an air pressure generator or pressuretank. The electric wires 51 and 58 are connected with an electric sourcefor heating the heating elements surrounding the muifle chamber. The

. air valve 14 is opened to permit an air flow through the tubes 13 and61 into and through the exhaust pipe 64. Depending upon conditions, theair valve 14 is more or less opened and the pressure within the tubes 13and 61 determined by the pressure gauge '.'2,which measures the pressurein the tubes 13 and 61. The now of pressure out of the tube 51 by way ofthe nozzle 63 creates a vacuum therebelow and draws the atmospherewithin the muiile chamber'through the aperture 88 and nipple 82 into theexhaust pipe 64 and forces same out to the atmosphere. To prevent thecreation of a vacuum in the mufile chamber, the door 35 is provided witha pair of apertures or openings H38 and these permit a continuous supplyof fresh air to the furnace, thereby providing a neutralizing oroxydizing atmosphere to develop in the furnace muiile chamber. At thesame time, should it be desired to pass some other gas into the mufllechamber besides fresh air, this may be done by supplying same throughthe openings |08 and having it drawn into and through the furnacechamber.

As is usual, it may be desired to vary the temperature within thefurnace, from a point at or below 1250 F., the usual low point for goldalloy castings, to a point of 1700 or 1800" F., the usual high point ofstandard stainless steel alloy castings, and for which purpose therheostat 41 is adjustable by the knob 4I. The temperature of the furnacecavity is at all times available on the pyrometer 98 through thethermocouple 88.

As was mentioned above, the furnace is used, first to melt out the waxpattern in the mold and in order to prevent this molten 'wax fromcontacting the heating elements in the furnace bottom inner liner 2|,there is placed within the furnace a removable metal tray I0 l.

From the foregoing, it will now be appreciated that there has beenprovided an electric furnace primarily for use in dental laboratoriesand the like, which adequately meets the objects initially set forthabove.

What is claimed is:

In an electric furnace of the class described, including an inclosingmetallic shell, a muilie substantially centrally of the shell, heatinsulat` ing lining between the shell and mullle and filling the spacetherebetween, a heating element associated with the muiile, means forcontrolling the current flow to the heating elementI said muille chamberhaving its one end open and its other end insulated by the lining fromthe shell back wall, pressure exhaust means connected with the mufliechamber through the back wall for exhausting gases and moisture from themuiile chamber, including an opening through the back wall, a nipplecarried by said back wall in line with the opening therein, an exhaustpipe connecting the nipple to the atmosphere, and a tube connecting apressure source with the exhaust pipe at a point above the nipple forcausing forced draft through the exhaust pipe, a valve carried by theshell for regulating the oW of the pressure through the tube and therebyregulating the force y of the exhaust draft and a gauge for indicatingthe pressure flow through the tube into and through the exhaust pipe.

AUGUST L. WEYHING.

